Optical waveguides in the form of thin, flexible optically transmissive fibers with an external reflective surface cladding are well known as optical fibers. Optical fibers are typically used as transmission paths for optical signals and in various instruments. Inertial rate sensors are one important application of optical fibers. In such uses, optical fibers are reliable, compact, highly resistant to electromagnetic interference, and offer increased sensitivity, bandwidth and data rates, as compared with other known configurations. With particular regard to inertial rate sensors, the use of optical fibers eliminates many of the problems of tight alignment tolerances associated with prior art bulk-optic systems.
One limitation of conventional optical fibers in many applications is that thermal changes in an optical fiber induce changes in the effective optical path length for optical signals propagating in the fiber, principally because the refractive indexes of the core and cladding are temperature dependant. As a consequence, the accuracy of fiber optic-based instruments is strongly dependent on the temperature stability of the optical fiber.
Optical fibers have been used as the light propagating structure in laser gyroscopes (or gyro's) in both interferometer and resonant ring configurations. In such systems, a generally circular fiber optic path is established and counter-propagating coherent light beams are established in that path. Rotation of the path about its central axis establishes different length effective optical paths for each of the counter-propagating beams. In a resonant ring laser gyro, in which the optical fiber provides a multiple turn closed ring path into which the two opposite-directed beams are coupled, one or both of the beams are adjusted in phase or frequency, typically in a closed loop manner, so that the counter-propagating beams establish standing waves in the ring. The phase adjustment provides a measure of the rotation of the ring about its central axis.
In an interferometer laser gyro, oppositely directed coherent beams are coupled into opposite ends of a multi-turn fiber optic open loop path. The output beams from the open loop path are then compared to detect interference effects which establish a measure of the rotation of the multi-turn loop about its central axis.
In the various gyro configurations, the measured rate is strongly dependent on ambient temperature. For example, small changes in effective optical path length in a fiber, such as might be due to changes in physical fiber length or diameter due to temperature gradients, can cause large errors in the measured input rate. Consequently, control of environmental temperature effects is desirable.
Known methods of attempting to control temperature effects on optical fibers include passive and active techniques. One passive method is the use of thermal insulation to either thermally isolate the optical fiber from temperature variations in its environment, or to prevent the environment from experiencing a temperature change. In the former approach, the optical fiber itself typically is wrapped with insulation. In the latter, insulating material is interposed between the instrument and its enclosure.
Active approaches to temperature control include the use of a heater block or oven. Typically, the fiber optic instrument is mounted on a heater block which is generally a temperature controlled metal mass. Alternatively, an oven may be used to directly regulate the immediate ambient temperature and constitutes a heated enclosure in which the fiber optic instrument is maintained.
These known methods have not been entirely satisfactory. Insulation has the disadvantage that the thermal stability it achieves varies over time, depending upon the thickness and type of insulation and upon the rate of change, range and extremes of the ambient temperature. In addition, the significant volume of insulation generally required in many applications to maintain thermal stability may exceed desired weight or space envelope limitations. Both of the described active control methods likewise share the disadvantages of significant weight and large space envelope requirements. Additionally, power consumption considerations can severely limit the use of active control methods in airborne or spaceborne applications.
Accordingly, an object of the invention is to provide an improved optical fiber which, in use, exhibits improved thermal stability.
It is a further object of the invention to provide improved light transmission characteristics of an optical fiber by reducing adverse thermal effects, while requiring minimal additional weight and space, and consuming minimal power.
It is yet a further object of the invention to provide a fiber-optic instrument such as a gyroscope which exhibits improved temperature stability.